1. Field of the Invention
The present invention generally relates to microlithographic printed patterns and structures and more particularly to improvements which decrease size differences between isolated and nested structures.
2. Description of the Related Art
As the minimum feature size in semiconductor integrated circuit technology is pushed near or below the wavelength of the light used in microlithographic projection printing, diffraction effects introduce significant differences between the patterns used on microlithographic reticles and the resulting printed structures on a semiconductor wafer. Similarly, the smaller the circuit elements become, the more difficult it becomes to create the desired pattern shapes on the wafer due to factors such as localized etch variations, mask distortions, lens distortions, topography variations, and non-uniform material composition.
These physical factors introduce deviations in the dimension of printed isolated structures versus printed nested structures, with the degree of deviation being highly dependent on the degree of proximity of nearby shapes. In order to maximize circuit performance and speed, it has been advantageous to make the device structure dimensions as identical as possible (e.g., to have isolated gates and nested gates print as identically as possible). These effects become increasingly important as the physical dimensions of the circuit elements decrease. However, it is difficult to make isolated and nested structures print as identically as desired resulting in an undesirable condition known as across chip line width variation (ACLV).
ACLV is a major problem in semiconductor device fabrication. Image size variations can affect transistor speed matching and resistivity and conductance matching from one portion of the chip to another.
It is, therefore, an object of the present invention to provide a structure and method for a photomask comprising a substrate having a first transmittance, a first pattern to be transferred to a photosensitive layer (the first pattern having a second transmittance lower than the first transmittance) and a second pattern having a third transmittance greater than the second transmittance and less than the first transmittance. The second pattern is adjacent to at least a portion of the first pattern, and the substrate and the second pattern transmit light substantially in phase.
The first pattern includes nested structures and isolated structures and the second pattern is adjacent an outer edge of nested structures and the isolated structures. The nested structures are spaced more closely than the isolated structures. The second pattern may be positioned between the nested structures.
The first pattern includes a pair of outer lines and at least one inner line and the second pattern is adjacent an edge of at least one of the outer lines or an edge of the inner line.
The invention also includes a method of preparing a photomask that includes supplying a substrate having a first transmittance, forming a first pattern to be transferred to a photosensitive layer (the first pattern having a second transmittance lower than the first transmittance) and forming a second pattern having a third transmittance greater than the second transmittance and less than the first transmittance. The second pattern is adjacent at least a portion of the first pattern, and the substrate and the second pattern transmit light substantially in phase.